1. Field of the Invention
The present invention relates to a camera having a function of predicting a future image plane position from a change in a plurality of past image plane positions and time of detection, a photographic lens to be mounted on this type of camera, and a camera system.
2. Description of the Related Art
Conventionally, in a camera system such as an interchangeable lens type single-lens reflex camera, a technique has been known, which combines a light flux from an object passing through different exit pupil areas of a photographic lens on a pair of line sensors. Consequently, an amount of image displacement can be determined which is a relative position displacement amount of a pair of image signals obtained by photo-electrically converting of an object image. As a result, a defocus amount of the object is detected from the amount of image displacement and based on the detected amount, the photographic lens is driven.
Further, many of these camera systems include a servo mode in which lens drive tracks not only a still object but also a moving object.
In a specific tracking method, an image plane position of an object (predictive position which may be focused on the object) is acquired from previously obtained two defocus amounts. In the tracking method, a difference between the image plane position of the object and a time of detection is obtained to determine an amount of displacement of the image plane position, that is, an image plane speed is calculated. Thus, the tracking method predicts the image plane position from a detection point after a predetermined time passes, to drive a photographic lens to the position.
Further, there is another tracking method. According to the tracking method, an image plane speed and an image plane acceleration, which is a change amount of the image plane speed, are calculated as a second order curve from previously obtained three image plane positions of an object and detection times. By the above-described calculation, an image plane position from a detection point after a predetermined time passes can be predicted.
Furthermore, a technique is described in Japanese Patent Application Laid-Open No. 2001-021794 in which the most suitable predictive function is selected by statistical processing.
These methods determine a predictive function based on a previously obtained temporal change of an image plane position of an object. The predictive function is extended over future time to predict an image plane position after a predetermined time passes, based on a detection point. Accordingly, the larger the difference between a predetermined time estimated when prediction is made, and the time when a release button is actually pressed and exposure is made, the lower the accuracy of the prediction. That is, it is important to accurately predict the time (also referred to as release time lag) between when the release button is actually pressed and when the exposure is started.
In an interchangeable lens type single-lens reflex camera, a factor that determines the release time lag exists on both of the camera (main body) side and the interchangeable lens side. On the camera side, the factor is the time (also referred to as mirror move up time) required after the release button is pressed until a mirror is retracted or a shutter is driven. On the interchangeable lens side, the factor is the time required to stop down an aperture of a diaphragm to a predetermined exposure (hereinafter, referred to as diaphragm driving time).